The present invention relates generally to collection devices for fashioning a tubular member of indefinite length from substantially continuous filaments of synthetic resinous material. More particularly, the present invention relates to configurations of a press roll which permit controlled radial density variation in the tubular product.
In the spray spinning art, synthetic resinous material is plasticated and pumped through filament shaping orifices to produce one or more filaments. Gaseous jets then act on the filaments to attenuate the filaments to a comparatively fine diameter and convey the filaments to a downstream collection device. Non-woven products of various configurations can be manufactured in this manner. For example, when a downstream collection device is a flat or curved surface or a set of parallel rolls, the resulting product is a generally a planar web of indefinite length. Another type of collection device which may be used with spray spinning apparatus is a rotating mandrel on which the filaments accumulate while the previously deposited filaments are withdrawn axially thereby producing a generally cylindrical member of indefinite length.
In the past, the production of cylindrical or tubular members of spray spun material has also been accomplished using a cylindrical mandrel and one or more generally cylindrical rolls which urge the tubular member axially off the mandrel. In some instances, these cooperating rolls may also compress the spray spun material during its accumulation on the mandrel. In either event, this cooperation between the press roll and the mandrel produces a tubular element having a radial density variation which decreases in a radially outwardly direction. In many applications for nonwoven tubular members it is desirable to have a uniform radial density variation, or a radial density variation which changes in a predetermined manner. Accordingly, the prior art devices have not been entirely satisfactory.
One specific example of a use for nonwoven tubular spray spun elements is a filter medium. If the filter medium has a high density on the fluid entering side, the particles will accumulate on the surface and prematurely clog the filter. Naturally, there are instances in which a predetermined radial density gradient in a filter medium is desirable: for example, where the particles to be filtered passes radially inwardly through the body, it may be desirable to have a density variation which increases in the radially inward direction. In this manner, larger particles accumulate in the lower density portions of the filter so as not to prematurely block the finer or higher density portions of the filter. Conversely, where a fluid passes radially outwardly through the filter medium, it will occasionally be desirable to have a density variation which increases in the radially outward direction. Similar considerations to those just discussed apply as to particle size and blockage for an outwardly increasing density variation.
It has been proposed to fabricate a cylindrical filter element with a radial density gradient by using a fiberizing assembly having a plurality of filament-producing orifices arranged in a row and inclined at an acute angle, with respect to the axis of the collection mandrel, see for example U.S. Pat. Nos. 3,933,557 and 4,021,281, issued to Pall. In this type of an assembly, the spacing between the filament-producing orifices may be varied and/or the angle at which the row of orifices inclined with repsect to the mandrel itself may be varied.
Such devices as those disclosed by the Pall patents do not, however, permit density variations to be created which exceed that naturally laid down by the spray spinning head. Moreover, the tubular product requires a separate device to physically withdraw the tubular product from the mandrel. Another difficulty with varying the orifice spacing and inclination angle is that the spray spinning mechanism must be changed or reoriented for each different density gradient. Such adjustments and orientation changes require substantial expense and consume much time since the plasticated material must pass through any adjustable or adjusted connection which is made.
U.S. Pat. Nos. 3,787,265 and 3,801,400, both assigned to the same assignee as the instant invention, show processes wherein weight changes in tubular idler rolls are used to compress the tacky fibers thereby inducing density variations in different layers of cylindrical self-bonded non-woven structures.
Accordingly, the need continues to exist for an economical and efficient means whereby the radial density gradient of a nonwoven tubular spray spun product may be varied at will with comparative ease.